SCARA (Selective Compliance Assembly Robot Aim) robots, in other words, horizontal multi joint robots, consist of, for instance, a base, an arm member, and a movable shaft. The arm member has one proximal end supported to the base mounted on an installation surface, such as a flooring of a factory. The movable shaft is supported to the other end of the arm member to be vertically movable and rotatable about a center axis thereof such that a higher end of the movable shaft projects from the arm member. To a lower end of the movable shaft, a gripper for gripping workpieces is attached.
An example of such SCARA robots is a SCARA robot using a ball-screw spline shaft as the movable shaft for vertically moving the gripper. Such a SCARA robot using a ball-screw spline shaft for vertically moving the gripper is for example disclosed in Japanese Patent Application Publication No. 2004-156735.
Specifically, a ball-screw nut and a ball-spline nut are rotatably mounted to the arm member away from each other in an axial direction of the ball-screw spline shaft such that the ball-screw spline shaft is threadably engaged with the ball-screw nut and ball-spline nut. The ball-screw nut and the ball-spline nut are electrically connected with an elevating motor and a swing motor, respectively.
Rotation of the ball-screw nut by the elevating motor is converted into vertical motion of the ball-screw spline shaft by screw threads formed on an outer cylindrical surface of the ball-screw spline shaft.
Rotation of the ball-spline nut by the swing motor is transferred to spline threads formed on the ball-screw spline shaft to thereby swing the ball-screw spline shaft.